Revert "Require Kotlin 1.6+"
Revert "Require Kotlin 1.6+"

This reverts commit c87f7b88bdd2db141afe7082ff5787e8e7329825. We moved a bit too fast, let's fix another dependency issue with Maven clients, first.

Relates to https://github.com/gesellix/docker-client/issues/245

docker-client/docker-filesocket,docker-client/docker-filesocket